The present invention relates to servo motor positioning system which may be used in any application in which it is required to accurately move a rotational load from a present position to a desired or command position.
An appparatus for producing an actual motor shaft angular velocity signal for use in such a system is disclosed in U.S. Pat. No. 3,819,268 entitled "VELOCITY DETERMINATION WITH OPTOELECTRONIC LINEAR POSITION TRANSDUCER". In one embodiment of this prior art apparatus a photoelectric transducer produces first and second periodic signals which are 90.degree. out of phase with each other in response to rotation of a servo motor shaft. The first and second signals are differentiated, full wave rectified and summed to produce a velocity signal having a magnitude proportional to the rotational velocity of the shaft. Full wave rectification is accommplished using gating signals derived from either the first and second signals or the differentiated versions thereof by means of voltage comparators.
This system is clearly advantageous over yet prior systems in which position signals are produced by a photoelectric transducer and a velocity signal is produced by a tachometer since the mechanical inertia of the components driven by the motor shaft is reduced and the response time is decreased. Also, the high cost of an electromagnetic tachometer is eliminated since the tachometer is replaced by low cost electronic circuitry.
The velocity signal produced in this manner is utilized in a servo positioning system by comparing the velocity signal with a velocity command signal which corresponds to the present angular distance between the actual motor shaft position and the command position. The difference between the velocity and velocity command signals is sensed to produce a difference signal which is applied to the motor.
While such a system is advantageously operable and provides generally acceptable service, the precision of operation thereof has been heretofore limited. The main problem involved is that the magnitude of the velocity signal is subject to variation in response to changes in the A.C. amplitude, D.C. component level and phase of the first and second signals produced by the transducer.
This problem is compounded by the voltage comparators used to produce the gating signals which typically comprise Schmitt trigger circuits. This is because the magnitude of the velocity signal is influenced not only directly as mentioned above but also indirectly since variations in the first and second signals also affect the relative trigger points in the comparators. Position errors are also produced since the first and second signals are used as position pulses and the leading edges thereof are subject to phase errors.
The difference or error signal applied to the motor is amplified by a servo amplifier. D.C. servo amplifiers known heretofore may be classified, according to amplification system, into a dropper type which performs continuous energization of the motor and a chopper type which performs intermittent energization of the motor. With regard to the type of switching system, D.C. servo amplifiers may be classified into a T type comprising two main switching elements and an H type comprising four main switching elements.
Each of the above described types of servo amplifiers have advantages and disadvantages. The H type is utilized where it is desired to drive the motor with low power and high speed. In a conventional chopper amplifier, the chopper frequency varies in accordance with the motor current and at very low current values may drop to such a low value as to be audible. In the prior art it has been necessary to use a special motor which does not have clicks and which is de-energized in the static or detent range. A conventional motor when so de-energized to stop the noise produced by the oscillation cannot enable accurate position control.
Another problem in D.C. servo motors is that counter EMF developed in the motor causes fluctuation of the power supply voltage upon reversal of the motor.